1. Field of the Invention
The present invention relates to an optical system and an image pickup apparatus having the same. The present invention is suitable, for example, for an image pickup optical system for use with a digital still camera, a video camera, a TV camera, and a surveillance camera, etc.
2. Description of the Related Art
An image pickup optical system used for an image pickup apparatus, such as a digital still camera and a video camera, is demanded for a small overall system and a high optical performance. Many image pickup apparatuses are made thin by retracting each lens unit in an image pickup optical system, in a camera body at the non-image pickup time. For a miniaturization of the retracted image pickup optical system, one known lens barrel has an aperture blade with a curved surface in an aperture diaphragm configured to vary its aperture diameter and located in an optical path. Japanese Patent Laid-Open No. (“JP”) 2007-94074 discloses a lens barrel that includes an aperture blade with a concave shape on the image pickup plane side for a concave surface of a front lens and a convex surface of a back lens with respect to the aperture diaphragm in the optical axis direction in the retraction state.
A known four-unit zoom lens has four lens units that includes, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. U.S. Pat. No. 7,830,613 discloses a rear focus type four-unit zoom lens in which each lens unit is moved for zooming, a fourth lens unit is moved so as to correct an image plane fluctuation associated with the magnification variation and to provide focusing.
Another known three-unit zoom lens includes, in order from the object side to the image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power. U.S. Pat. No. 7,023,623 discloses a three-unit zoom lens in which each lens unit is moved for zooming and the third lens unit provides focusing.
When a light quantity incident upon an image plane is restricted in the image pickup optical system by changing an aperture diameter of an aperture diaphragm, it is important to properly shield the light so that an incident light amount can change in a well-balanced manner over the entire image. A ratio of the brightness between the center and the periphery of an image becomes too high and a difference between the brightness and darkness stands out, if the light intensity incident upon the periphery of the image significantly drops when the aperture diameter of the aperture diaphragm is changed. It is thus important to change the aperture diameter of the aperture diaphragm according to a light flux incident upon the aperture diaphragm.
It is difficult to obtain a good image when the axial position of the aperture part of the aperture diaphragm does not change according to the aperture diameter because the best axial position of the aperture part of the aperture diaphragm changes in the image pickup optical system according to the aperture diameter of the aperture diaphragm.